The Stirling engine was invented by the Reverend and engineer Robert Stirling in the early 19th century as an alternative to steam engines which were very dangerous because of the numerous explosions thereof.
A Stirling engine is a machine which operates on a closed regenerative thermodynamic cycle with periodic compression and expansion of a working gas at different temperature levels. It has pistons moving in cylinders but the movement of the pistons is not caused by a pressure variation due to the combustion of a gas as in the internal combustion engine. Instead, the Stirling engine uses a working gas circulating in closed circuit between one or more pairs of pistons. The flow of the working gas is controlled by volume changes in such a way as to produce a net conversion of heat to work. The heat is provided by an external heat source allowing either continuous combustion of various liquid, gaseous or even solid fuels, or either the use of solar, geothermal or waste energy from another process or engine.
Nowadays, there are two main embodiments to the Stirling engine. The first one comprises free pistons generating a linear movement. The second one comprises a crankshaft operatively attached to a piston by a connecting rod. The movement of pistons may be directly transmitted to the crankshaft or be transmitted to a diamond-shaped driving mechanism mechanically connected to two counted-rotating crankshafts. None of these Stirling engines is mass produced mainly because they are complex and because of their uneconomical cost. Additionally, the movement of the pistons in these engines is also continuous.
Although the Stirling engines are not very common on the market, they have many advantages. They allow very low concentrations of pollutants, a quiet running without combustion noise and a fuel consumption which roughly corresponds to diesel engines at equivalent speeds.